FIG. 7 is a front view of a prior-art camshaft driving mechanism. In a four-cycle engine, parallel camshafts 50 and 52 must rotate at one-half of the speed of a crankshaft 53. Therefore the outside diameter of camshaft wheel members 54 and 56, which are driven by a power transmitting member such as a chain 59, is twice as large as that of a driving crankshaft wheel member 58. The cylinder head (not shown) for the engine thus becomes larger with an increase in the size of the camshaft wheel members 54 and 56, and inevitably results in an increase in the total weight of the engine.
As a solution to the above-described problem, there have been proposed the arrangements shown in FIGS. 8 and 9. In FIG. 8 the power transmitting member 59 drives only the crankshaft wheel 54, and the camshafts 50 and 52 mount thereon smaller diameter driving wheels 64 and 66 which are drivingly coupled through another flexible power transmitting member 62. In FIG. 9, the crankshaft driving wheel 58 drives an intermediate wheel 80 through the first power transmitting member 72, a further smaller-diameter intermediate wheel 82 is coaxially driven from wheel 80, and wheel 82 drives a second power transmitting element 78 which in turn drives the smaller diameter wheels 74 and 76 which are secured to the camshafts. These solutions, however, tend to give rise to other problems, i.e., an increase in the number of component parts, and accordingly an increase in the weight and thickness of the engine.
The present invention has solved the problem stated above by adopting an arrangement wherein the camshaft driving mechanism has the two camshafts in the cylinder head and drives the camshafts through a power transmitting member mounted between the crankshaft wheel member and the camshaft wheel members which are fixedly mounted on the camshafts. In this camshaft driving mechanism, the thickness of at least one of the camshaft wheel members is one-half or less of the width of the power transmitting member, one of the camshaft wheel members is offset in the axial direction from the other camshaft wheel member, and the camshaft wheel members partly overlap one anther in the radial direction, or one of the camshaft wheel member has a center slit in the axial direction while the other camshaft wheel member fits in the slit.
According to the present invention, the two camshaft wheel members are driven by the power transmitting member installed between these camshaft wheel members and the crankshaft wheel member. Since the camshaft wheel members are radially overlapped without interfering with each other, there is a short center distance (i.e. the perpendicular distance or spacing) between the two camshafts. If is, therefore, possible to use a small cylinder head to thereby reduce the total weight of the engine.
Hereinafter an exemplary embodiment of the camshaft driving mechanism according to the present invention will be explained with reference to the accompanying drawings.